Canopy nitrogen distribution and the photosynthetic performance of sunflower crops during grain filling ? a quantitative analysis

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Abstract
Measurements of the profiles of leaf area and leaf nitrogen were made on five occasions from midflowering to maturity (53, 61, 70, 78 and 83 days after emergence, DAE) in sunflower crops grown at contrasting density (2.4 and 4.8 plants m-2) and nitrogen supply (0 and 5 g N m-2 at emergence) in the summer in Buenos Aires, Argentina. As the crops matured, nitrogen was withdrawn unequally from all leaf positions and leaves senesced from the bases of the canopies. A model was used to estimate the daytime net photosynthesis (Pc) of canopies of defined leaf area and nitrogen content under the observed conditions of temperature and irradiance. Comparisons were made between the observed profiles of leaf nitrogen and those that would maximise Pc (the optimal profiles). The observed nitrogen profiles were sub-optimal at mid-flowering, except in the low-density, low-nitrogen treatment. The differences were most marked in the high-nitrogen treatments which held ‘excessive’ nitrogen in their lower canopies. As the canopies matured and nitrogen was mobilised to the grain, leaf area index and total nitrogen content decreased and optimal profiles changed shape from exponential to linear. During this period observed profiles became more optimal. There was, however, little difference in Pc between observed and optimal profiles. The maximum difference was 3.2% observed in the low-density, high-nitrogen treatment at DAE 53. The comparison of actual and optimal profiles as leaf nitrogen content (mg N) in addition to the more commonly used specific leaf nitrogen (SLN, g N m-2 leaf) explains this result because relatively large changes inSLN in the small leaves at the top of canopies have little effect on Pc. The study shows that leaf nitrogen content is an appropriate basis for comparison of canopy nitrogen profiles in sunflower.